Chapter 4
Light that appears white to our eyes is emission from a blackbody at ~6000K (our Sun!)
See Figure 2.8
The sensitivity of our eyes to light follows very closely the emission spectrum of the sun!
why?
Light traveling through the atmosphere can be:
Before
After
Absorbed
Transmitted
Reflected
(angle of incidence = angle of reflection)
Air
You usually see reflection
at a flat, smooth interface –
This is Snell’s Law
Glass
If the interface is rough (on a
scale of λ = 0.5µm), you get
reflection in each microscopic
area, but overall, reflected light
goes in all directions.
This is diffuse scattering .
You also get
diffuse scattering
off of a collection
of particles.
A particle’s probability of scattering light depends on how well the size of the particle
matches the wavelength of light.
Scatters all
wavelengths
evenly
1 µm particle
Selectively
scatters red
(Mie
scattering)
< 0.1 µm air
molecule
Selectively scatters
violet, blue, green
(Rayleigh scattering)
So, clouds don’t change color of light, they just attenuate light.
20µm water drop
in a cloud
All
Orange
Blue
Figure 4.8
Whereas air molecules selectively scatter light of short wavelength, like violet, blue and
green.
Refraction:
normal
normal
2 rules of refraction:
1. Light traveling from a less dense medium
to a more dense medium bends toward the normal.
(And vice versa)
air
water
Light is reflected at mirror.
For glancing angles, bending is most exaggerated,
while for normal incidence, no bending.
mirror
2. Short wavelengths (violet, blue) are bent more than long wavelengths (red). (This is
called dispersion)
Example: green flash
green
violet
normal
red
red
green
violet
Violet is not intense.
Blue is absorbed.
Green makes it to eye.
Note:
In celestial navigation, a correction must be applied to correct star position for
refraction.
If your time reading is off by 4 sec from universal time, you are off your mark by
~1 mile!
4 sec in time ≅ 1 minute of arc (1/60° of arc)
Mirages:
Light passes through layers of air having different densities because of temperature
variation, and refracts.
Example: “wet” road
Blue sky
Light bends away
from normal
Looks like
sky is in road
Hot road
Rainbows:
Total internal reflection
At some critical angle, θc , refracted beam is 90° from normal.
At angles above θc , all light is reflected back into denser medium and there is no
refracted beam.
This is the principle of fiber optics.
All light stays inside fiber and none escapes through refraction (no losses).
In rainbows, light incident on water drops at angles greater than θc is reflected and comes
back toward the observer. Otherwise, water would be mostly transparent!